This invention relates to a superconducting apparatus having a superconducting solenoid coil comprising a thin coil wound around a bobbin, and particularly to means for suppressing local increases in the magnetic field in the end region of the superconducting solenoid coil.
FIGS. 1 and 2 illustrate a conventional apparatus of this type. In the figures, a superconducting solenoid coil 1 is wound into the form of a cylinder by winding a superconducting wire 4 around a cylindrical bobbin 5. The superconducting wire 4 comprises a superconductor 2 embedded within a stabilizing material 3. The superconductor 2 has a rectangular cross section with a thickness which is much less than its width. The superconducting solenoid coil 1 is housed within a cryostat 6.
The operation of this conventional apparatus is as follows. The cylindrical thin superconducting solenoid 1 having a coil thickness which is small compared to its diameter is utilized in a particle colliding apparatus for studying elemental particles of high kinetic energy as discussed in the article "CONSTRUCTION AND TEST OF THE CELLO THIN-WALL SOLENOID" (1980, Adv. Cryog. Eng. 25, p.175-p.184). The coil is made as thin as possible to make it more transparent to elemental particles. The materials used for constructing the apparatus are based mainly on aluminum and carbon, except for the superconductor 2, taking better transparency to particles into consideration. As a matter of course, an extremely high current density is used in order not to unnecessarily increase the sectional area of the superconductor 2. During the operation of the superconducting solenoid coil 1, current flows only through the superconductor 2 of the superconducting wire 4 and usually does not flow through the stabilizing material 3. The current flows through the stabilizing material 3 when the superconducting state is destroyed and the current bypasses to promote a return to a superconducting state as described in "Institute of the Electrical Engineering Collegiate Lectures; Superconducting Engineering" (1974, Japanese IEEE, P.60-P.65). Thus, since the superconductor 2 has a very small cross sectional area compared to the diameter of the coil 1, a very strong magnetic field is generated at the end portions of the superconducting solenoid coil 1. This is one kind of an end effect and a similar phenomenon is discussed in "Electromagnetic Phenomenon Theory" (S. Maruyama, Maruzen Press, 1944, p. 184 ) The electric field strength .sigma. at the end portion of a semi-infinite plane is expressed by ##EQU1## which equals ##EQU2## Accordingly, when x=0, .sigma.=-.infin.. When the thickness is infinitely small, the magnetic field at the end portion of the superconducting solenoid coil 1 becomes infinitely large. While the magnitude of this magnetic field generally has an upper limit due to the finite thickness of the coil, it nevertheless reaches a significantly high value.
Since the conventional superconducting apparatus is constructed as above described, an increase in the magnetic field at the end portions of the superconducting solenoid coil is inevitable, which sometimes makes superconductivity impossible since the upper limit of the current depends upon the strength of the magnetic field experienced. More particularly, even when sufficient stabilization is provided in terms of maintaining the superconducting phenomenon, once partial destruction of the superconductivity occurs, the destruction spreads in a chain-reaction. Therefore a reliable counter measure is necessary for a large, high energy experimental apparatus such as that described above.